1. Field of the Invention This invention relates to a process for producing industrially usable gypsum from a commercial by-product, in particular fluoroanhydrite. More particularly, this invention relates to an improvement in processes for the production of hydrogen fluoride and waste product fluoroanhydrite.
In the industrial production of hydrogen fluoride, fluorspar is reacted with concentrated sulfuric acid in an externally heated reaction vessel to co-produce hydrogen fluoride and fluoroanhydrite. For every ton of fluorspar consumed, approximately 1.75 tons of anhydrite is produced. After reaction, the hydrogen fluoride is drawn off and condensed while the fluoroanhydrite is generally slurried with water, neutralized with lime or limestone and pumped to disposal ponds.
The fluoroanhydrite is contaminated with excess sulfuric acid and fluoroaluminate impurities. Historically, because of the contaminants, the fluoroanhydrite has not been commerically useable and has been allowed to hydrate naturally over a several year period of time. Thus large tonnages of mixed fluoroanhydrite-fluorogypsum materials have been accumulated in these ponds over the years.
2. Description of the Prior Art
U.S. Pat. No. 3,825,655 discloses an improvement in hydrogen fluoride processes said to produce a coarse-grained sulfate co-product in either anhydrite or gypsum form said to be industrially useful.
However, it has been shown that certain fluoroaluminum impurities still present, e.g. [(AlF.sub.5 H.sub.2 O)].sup.-2 cause deleterious effects in attempts to manufacture gypsum products from the fluoroanhydrite. These impurities inhibit the hydration of the fluoroanhydrite to gypsum. These impurities raise the calcining temperature during conversion of fluorogypsum to calcium sulfate hemihydrate. Further, they inhibit the setting of the produced hemihydrate; and finally, they result in producing gypsum products such as industrial plasters, building plasters and gypsum wallboard of poor quality.
From the above, it is apparent that there is a need in the art for effective and economical means for removing a substantial portion of the impurities that originate in the hydrogen fluoride process. Furthermore, there is a need in the art for making calcium sulfate products that may be used in wallboard and other industrial and construction materials. The process of the present invention offers a solution to these needs.